A Treasury study asserts that less-developed countries have become more receptive to explorations by private oil companies since recent oil-cartel price increases, diminsihing the need for a new World Bank affiliate to encourage Third World oil development.

In an interview last week in which he reveled the existence of the study, Treasury Secretary Donald T. Regan said that "if there is a pool of oil of any commercial value, the oil company that found it will exploit it, provided that it can reach a satisfactory return on its money."

Regan said that oil development in the Third World by the private sector has been frustrated in the past because the countries and companies hadn't been able to agree on terms. But a number of countries "which previously were reluctant to admit foreign companies are now permitting exploration," he said. Regan cited as examples India, Brazil, Argentina, Turkey and Chile.

The United States thus intends to convey to its partners of the Ottawa summit next week its belief that the energy affiliate many of them favor is not needed. At the same time, Regan said the Reagan administration will encourage the World Bank to expand its energy-development role within its present structure. This appears to be a softening of a position taken earlier in the Reagan administration.

Moreover, Regan said that "without committing ourselves, we're willing to explore" the possibility of liberalizing the bank's "gearing ratio," not only for energy but for all bank lending operations. Presently the bank's gearing ratio allows only a dollar-for-dollar lending total in relation to capital.

"What we're saying on the energy affiliate is that this is something that we don't think has to be set up as a separate [entity] with separate funding and a separate staff," Regan said in an interview. "We think much of it can be done within the charter, the capital and the borrowings of the World Bank right now. What most of these countries and oil companies need is a middleman, if you will, somebody to divide the profits, to guarantee against exploitation on both sides."

Regan added that much of this can be done within the present structure of the World Bank, which should traget its energy-development program in the Third World not only to crude oil, but to hydroelectric power, biomass and reforestation. "We tend to forget that in many nations, particularly in the tropics, wood is their chief source of energy," the Treasury official said.

Backers of an energy affiliate for the World Bank have argued that the private multinational oil companies are not anxious to get involved in many parts of Asia and Africa unless they are assured there will be significant exportable quantities of oil. Bank sources said this effectively ruled out private exploration for amounts of oil that would be important for local consumption.

Not so, said Regan: "If the price is right" and "the private oil companies find . . . enough oil in a country to satisfy that country's needs, and the price of extracting the oil and the sale price at the pump or wherever it's being done warrants it, it can be done. Now what you've got to remember is that if it's a small pool of oil,, it would hardly pay to extract that oil and the process it in that country."

The Treasury study shows that in the 10-year period 1967-76, private companies explored for oil in 93 out 113 nations that are not members of the Orgtanization of Petroleum Exporting Countries. The 20 countries with no private oil-company activity at all are mostly small island nations where the geological prospects were considered poor.

The analysis goes on to point out that because actual activity occurs with a planning lag of as much as five years, most of these explorations were blueprinted or undre way before the first oil price shock of 1973-74, even at a time when oil prices were no more than a tenth their present level. Thus, the study suggests that vastly higher prices now, combined with an improved investment climate in many of the countries, will accelerate the exploration process.